Anode for electrochemical applications



M6319, 1964 w. l.; MILLER r-:TAL 3,133,872

ANODE F'OR ELECTROCHEMICAL APPLICATIONS Filed March lO, 1959 f 20a Q United States Patent O 3,i33,872 ANGDE FOR ELECTRCHEMHCAL APPLICATINS Walter L. Miller, Lynhroolr, NY., Herman S. Preiser, North Springiieid, Va., and Sidney Tudor, Forest Hills, NKY., assignors to Chemionics Engineering Laboratories, lne., a corporation of North Carolina Filed Mar. 1d, 1959, Ser. No. 798,567 l. Qlaim. (Cl. 21M- 196) (Granted under Title 35, US. Code (1952;), sec. 266) The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.

This invention relates to a lower cost anode for electrochemical applications for serving substantially the same functions now served by high cost anodes made of highcost metals such as platinum or alloys thereof selected for the properties of high conductivity, inertness in the selected electrochemical application, and low contact resistance relative to the electrolyte wherein the anode is immersed during the electrochemical application.

More particularly this invention relates to a lower cost anode for cathodic protection to resist corrosion of ship hulls, tanker ship compartments, tanks for storage and transportation, petroleum and chemical process and storage equipment, fixed or mobile structures such as docks, piers, and canal locks immersed in sea Water, fresh water or brackish water, structures such as pipelines, foundations, and storage tanks buried in soils. This invention relates to anodes of any conguration known in the art and for support in any of the variety of ways known in the art. ln a cathodic protection application, an anode is connected to the positive terminal of a direct current source, such as a rectifier, battery, or generator, and the surface to be protected is connected to the negative terminal as the cathode. As a result of the current flow, the cathode is polarized and the flow of corrosive current is reduced or eliminated with consequent reduction or elimination of the corrosion.

The problem `solved by this invention is readily appreciated by surveying the types of anodes used for an electrochemical application such as cathodic protection. Gne type that is used for this purpose is a solid platinum anode. Platinum does not erode significantly in a cathodic protection application because of its high resistance to electrolytic oxidation in most electrolytes, including sea water, and it has excellent conductivity; however, its cost is staggering. Anodes of platinum, coated, clad, or plated on cores of a good conductor such as copper, steel, brass, or silver, have been used. The latter are far less expensive than solid platinum anodes but are not reliable. If there is any discontinuity such as a pore in the platinum coating, the core metal is rapidly eroded by anodic current during electrochemical action. If the platinum layer is made thick enough to substantially preclude exposure of the core to the electrolyte, the anode is still very expensive.

An object of this invention is to provide an anode for electrochemical applications that is just as effective and long-lived as high grade anodes now in use but of substantially lower cost.

Other objects and many of the attendant advantages of this invention will be readily appreciated as the saine becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings wherein:

FIG. l shows partially in plan and partially in section a conventional anode configuration which may embody this invention,

FIG. 2 is a section taken along line 2-2 of FIG. l and wherein the anode is constructed in accordance with one embodiment of this invention,

FIGS. 3 and 4 correspond to FIG. 2 and illustrate two other embodiments of this invention,

FIG. 5 showspartially in plan and partially in section another anode embodying this invention, and

FIG. 6 shows in section another anode embodying this invention.

This invention is an improved composite anode. In its broader aspects it includes a thin exposed layer of a good conductor that is substantially inert before, during, and after an electrochemical application in a selected electrolytic medium and has low contact resistance relative to that medium. The thin exposed layer may include discontinuities ranging in size from minute pores to a substantial percentage of the exposed area. The layer may be as thin as several millionths of an inch. Because materials having the necessary characteristics of inertness, good conductivity, and low electrical contact resistance relative to an electrolyte, are very expensive, the layer of this material is as thin as possible consistent with factors such as desired useful life, likelihood of erosion by abrasion, and current density. The remainder of the composite anode is of substantially less expensive material than said exposed layer and includes a conductor that mechanically supports and makes good electrical contact with said exposed layer. The surface of the conductor supporting the exposed layer is a metal of the type that is very resistant to erosion under these conditions. A metal that forms a film of high electrical resistance wherever its surface is in contact with the selected electrolyte during electrochemical action and resists erosion thereof in the electrolyte, is excellent for the purpose. The interface between the exposed layer and the supporting conductor is for the most part free of any such ilrnl and of any other poorly conductive impurity so that there is low contact resistance at the interface. If, for the sake of economy, the supporting conductor is composite and comprises predominantly a body of a conductive material that will suffer erosion if it carries anodic current when it is in direct contact with the electrolyte, and the surface of the supporting conductor is a protective film forming surface as above, the protective surface material must be continuous wherever it is likely to become exposed to electrolyte and its thickness must be such that it will withstand anticipated mechanical abuse without exposing the conductive material beneath to erosion by the electrolyte; also the interface therebetween is sufficiently free of nonconductive film or other poorly conductive impurities to insure adequate electrical conduction.

In FIG. 1, there is shown a cylindrical anode 10, a conductor 12 electrically connected to one end thereof, with that end of the anode embedded in an insulating body of material 14 for supporting the anode and insulating the electrical connection. FIG. 2 shows in cross-section the construction of the anode of FIG. 1 in accordance with one embodiment of this invention. The anode includes a rod-like core 16 of a metal which is substantially rigid and which is very resistant to erosion wherever exposed to the electrolyte in which the anode is to be used for an electrochemical application. For cathodic protection applications in sea water for instance, tantalum and titanium are very satisfactory metals for core 16.V Both these metals have appreciable natural corrosion resistance; in addition, these metals form a tough surface iilm wherever exposed to the electrolyte during an electro-chemical application. This iilm has high electrical resistance to anodic current and practically stops anodic current flow therethrough. Tantalum is superior to titanium in that it functions satisfactorily at substantially higher voltages than titanium can. Titanium is less expensive than tantalum. The core 16 is coated by a thin layer 18 of metal that is a good conductor, presents low contact resistance to current ilow between itself and the electrolyte, and is substantially inert in the electrolyte in which the anode is to be used. This metal may be one of the following: platinum, platinum-palladium alloys, patinum-indium alloys, indium palladium, gold, rhodium, and other noble or inert metals. The choice of metal varies with conditions under which the anode is to be used, such as the specific electrolyte, current density, likelihood of mechanical abrasion, desired life and other practical considerations. Platinum has been successfully used in cathodic protection anodes in sea water. Since the above-listed metals are expensive, the layer 18 is made as thin as possible consistent with the above considerations. It is not necessary that the layer 18 be continuous; the important consideration is that there be sufficient surface area to carry the anodic current and without suffering deterioration. The layer 18 is put on the surface of core 16 in substantially intimate face to face contact therewith by cladding or other methods known in the art. The interface between the outer layer 18 and the core 16 must be substantially free of nonconductive film or foreign material that would introduce substantial contact resistance. In this form of the invention, the conductor 16 may be tubular rather than solid but with sufficient thickness to satisfy mechanical rigidity and strength requirements. Cladding and subsequent expansion by rolling before forming the tube can break up nonconductive film and thereby ensure low contact resistance at the interface.

In the embodiment shown in FIG. 3, the erosion resistant metal corresponding to the core 16 in FIG. 2 is made in the form of a thin layer 16a to reduce cost, since materials having the necessary erosion resistant characteristics like tantalum and titanium are expensive though substantially less so than platinum. The thin layer 16a envelops a rod 20 of comparatively low cost metal such as cooper, brass, lead, steel, or aluminum. The layer 16a is continuons, that is, impervious to electrolyte in which the anode is to be used, over at least that portion of inner rod 20 that may come in contact with the electrolyte in an electrochemical application; if the rod 20 is exposed to the electrolyte it will erode rapid- 1y. The interface between inner rod 20 and layer 16a is substantially free of nonconductive film or foreign material to ensure low contact resistance. The methods of making this embodiment are the same as for FIG. 2.

If the anode is not required to carry much current or needs to be lighter in weight than a solid metal anode, for a particular application, a core of synthetic resin can be substituted for the metal core in FIG. 3. This is shown in FIG. 4. This core 22 consisting of a nonconducting, filled or unfilled epoxy, polyester, or phenolic resin having adequate resistance to electrolytic oxidation products may be cemented to or cast with the bonded metal layers 16a and 1S. Alternatively, where the layer 16a is a rigid tube, the nonconducting core 22 may be provided by pouring in the uncured resin and then curing the resin core. An advantage of this embodiment is that if the two layers 16a and 18 are penetrated as by a sharp object during an electrochemical application, the core 22 will be unaffected. In the embodiment shown in FIG. 3, if, by chance, the metal core 20 is exposed by a puncture through the layers 16a and 18, the core 20 will erode.

In the embodiment shown in FIG. 5, a metal layer 18a corresponding to layers 18 in FIGS. 2-4 is on a surface of bar 16h but does not surround the bar as in the embodiment in FIG. 2. The material of bar 16b corresponds to the material of rod 16 in FIG. 2.

In the embodiment shown in FIG. 6, the anode is disk shaped, or bar shaped as in FIG. 5, but the erosive resistant material is a thin layer 16C as in FIGS. 3 and 4. The two thin layers 16C and 18a are supported by a layer 20a corresponding to bar 20 in FIG. 3. Since the metal layer 20a will erode if it contacts the electrolyte, an electrolyte impervious resin insulating body 14a correspondi 'mg to insulator 14 in FIGS. 1 and 5 shields the layer 20a and the electrical connection to conductor 12b against erosion by the electrolyte.

In the several embodiments, if 18 or 18a is of platinum and 16, 16a, 16b, 16e is tantalum, the anodes have the advantages of a comparable solid platinum anode but cost considerably less. Such anodes have high resistance to chemical and electrochemical deterioration provided by the platinum and tantalum, low metal to liquid electrical resistance provided by the platinum 18, 18a, and a current carrying capacity determined by the entire metal cross section and by the exposed surface area of the platinum. In the several embodiments the anode is provided with the necessary mechanical strength propcities by the comparatively lower cost material supporting the platinum layer. The comparatively lower cost metals supporting the platinum layer contribute to the current carrying capacity of the anode.

In the several embodiments described, substantially all the anodic current passes to the electrolyte by way of the layer 18 or 18a, which continuously retains its low electrical surface resistance.

Obviously many modifications and variations of the present invention are possible in the light of the above teachings. It is therefore to be understood that within the scope of the appended claim the invention may be practised otherwise than as specifically described.

We claim:

A rigid, cathodic protection anode for use in aqueous conductive liquids comprising: an exposed first material having characteristics of inertness, good conductivity and low contact resistance relative to aqueous conductive liquids, for electric current transfer between the anode and aqueous conductive liquid in which it is immersed, and a base or core for the first material formed of a second material having the characteristic of forming a film of high electrical resistance wherever its surface is in contact with the aqueous conducting liquid and resisting erosion in the aqueous conductive liquid, said second material being in the form of a thin walled supporting member for the first material and of sufficient thickness to withstand mechanical abuse in cathodic protection applications, said first material overlying at least the major portion of the surface of the thin walled member likely to be exposed to contact with and wetting by the aqueous conductive liquid, said first material being a thin layer, in intimate face to face physical contact therewith, with low electrical resistance therebetween, the interface between said two materials being substantially free of film and foreign material, and a rigid element being substantially stronger than the combination of thin walled element overlaid with said first material, and intimately engaging and firmly supporting the entire thin walled member during cathodic protection applications with the thin Walled member disposed between the first material and the rigid element, said rigid element being of nonconductive material that does not erode in the conducting liquid, said thin walled element and said rigid element being in intimate face to face physical contact.

References Cited in the file of this patent UNITED STATES PATENTS 1,477,099 Baum Dec. 11, 1923 2,222,979 Lemaire Nov. 26, 1940 2,508,171 Kaufman May 16, 1950 2,719,797 Rosenblatt et al Oct. 4, 1955 2,795,541 Muller June 1l, 1957 2,863,819 Preiser Dec. 9, 1958 3,038,849 Preiser June 12, 1962 FOREIGN PATENTS 904,490 Germany Feb. 18, 1954 OTHER REFERENCES Cotton: Platinum Metals Review, vol. 2, No. 2. April 1958, pages 45-47. 

